The long term objectives of this work are to contribute to the elucidation of the normal structural organization and development of the vertebrate visual system, with an emphasis on characterizing the mechanisms that govern the development of connections between the eye and central visual centers. The issues to be addressed by the proposed studies are of broad significance. The experiments will be carried out principally using fetal and postnatal rats, and in chick embryos, combining in vivo and in vitro approaches. As experience has shown, though, the developmental mechanisms identified and characterized in the avian and rodent visual system also operate in the development of the primate visual system. The first of the 4 major aims of this proposal concerns the process of target selection by developing retinal axons. Aims 2 - 4 characterize the mechanisms employed by retinal axons to develop topographically ordered projections within their principal target structures, the superior colliculus, and its avian homologue, the optic tectum. AlM 1. To characterize in vivo the mode of in growth of rat retinal axons in to the dorsal lateral geniculate nucleus and test in vitro the hypothesis that a target-derived chemoattractant controls the process by which retinal axons select this nucleus for innervation. AIM 2. To define for the rat retinocollicular projection the temporal sequence of emergence of topographic order from an initially diffuse projection and determine whether topographically aberrant arbors transiently form synaptic contacts. AIM 3. To characterize in rodents the early targeting of developing retinal axons along the rostral-caudal axis of their primary target, the superior colliculus, and assess using an in vitro assay whether this targeting and subsequent arborization of retinal axons reflect their response to regional-distinctions in molecular cues present on the surfaces of collicular cells. AIM 4. To evaluate in vivo the growth behavior of developing chick retinal axons to determine their response to putative molecular cues that encode position along the rostral-caudal and medial-lateral axes of their primary target, the optic tectum.